AFP is a plasma protein that is normally produced by the fetus. It serves as the basis for some valuable tests. AFP is manufactured principally in the fetus's liver and gastrointestinal (GI) tract besides the yolk sac, a structure temporarily present during embryonic development. The AFP level is typically high in the fetus's blood. It decreases in the baby's blood after birth. And by one year of age, it is virtually undetectable. During pregnancy, AFP crosses the placenta from the fetal circulation and appears in the mother's blood. The AFP level in the mother's blood (the maternal serum AFP) provides a screening test for several disorders including: Open neural tube defects (anencephaly and spina bifida); and Down syndrome (and other chromosome abnormalities). The maternal serum AFP (MSAFP) tends to be: High with open neural tube defects such as anencephaly and spina bifida (meningomyelocele); and Low with Down syndrome (trisomy 21, an extra chromosome number 21). AFP production is essentially zero after a year of age. However, it increases again under the stimulus of some liver diseases. It may, for example, be produced by viral hepatitis and cirrhosis of the liver. AFP is also made by primary liver tumors (hepatomas) and by germ cell tumors (teratocarcinoma and embryonal cell carcinomas). A person's serum AFP level can therefore be used to help detect these conditions and monitor their treatment.